Abstract

Geographical variation refers to differences among populations in genetically based traits across the natural geographic range
of a species. Understanding the factors that give rise to and maintain geographical variation helps elucidate the causes and
consequences of evolution. In the simplest case, we divide these factors into purely genetic versus environmental components
to tease apart their relative contributions to observed phenotypic variation. Subsequent experiments allow us to partition
further the genetic components of phenotypic variation towards a molecular understanding, hopefully to reveal the actual deoxyribonucleic acid (DNA)‐scale changes giving rise to adaptations we see at the population level. Studies of a select number of organisms –
all of which begin with geographic variation – demonstrate this highly productive strategy. Given its relevance to differentiation
among populations, geographic variation also has direct bearing on the question of species origins.

Key Concepts:

Geographical variation refers to differences among populations of organisms in genetically based traits across the natural
geographic range of that species.

Understanding how initial population size, gene flow, mutation frequency, and environmental (abiotic) factors give rise to
and maintain geographical variation can shed light on direction and rate of evolutionary change.

Reciprocal transplantation and ‘common garden’ experiments are two effective manipulations that can reveal genetically based
phenotypic variation.

A number of studies of geographic variation, including those about sickle‐cell anaemia, warfarin‐resistance, and lepidopteran
colouration, have shifted to a more molecular focus.

A number of other studies attempt to understand geographic variation in the broader context of community and ecosystem structure.

Given the multi‐faceted approach to understanding it, geographic variation can help address the question of species’ origins.

Keywords: adaptation; drift; clines; selection; speciation

Figure 1.

The effects of population size on random genetic drift. Allele frequencies for six different genes are plotted from the results
of a Monte Carlo random simulation. Simulation models permit investigation of evolutionary change when random influences (e.g.
genetic drift and founder effects) may be important. Although each run of the simulation model will produce slightly different
results due to differences in random effects, overall mall populations lose genetic variation rapidly and are subject to rapid
shifts in gene frequencies from one generation to the next. Large populations, in contrast, appear more resilient to the effects
of random genetic drift. These models were generated using ‘Populus’ (http://biosci.cbs.umn.edu/software/populus.html), a simulation program available from Donald Alstad at the University of Minnesota. Initial allele frequencies were 0.5 for
all genes, and simulations were run for 100 generations for population sizes of 20 (top) and 100 (bottom).

Figure 2.

Founder effects and population differentiation. Founded by one or a few individuals, ‘satellite’ populations have different
allele frequencies from the more genetically heterogeneous source (and from each other) as a result of founder effects. Founder
effects may play a role in rapid divergence and speciation because they sometimes lead to combinations of genes of superior
fitness that could not persist in the larger source population.

Figure 3.

The effects of migration on genetic variation within and among populations. In clusters of small populations, migration among
populations tends to eliminate genetic differentiation and increase genetic variation within populations (top). In the absence
of immigration, genetic drift may lead to differentiation among populations and the loss of genetic variation within populations
(bottom).

Figure 4.

Clinal variation in body size. A cline forms when a trait shows gradual change along an environmental gradient (e.g. latitude
or altitude). Many endotherms (e.g. birds and mammals) show an increase in body size from south to north – a pattern of variation
known as Bergmann's rule. In contrast, many ectotherms (e.g. insects) show the ‘converse to Bergmann's rule’, whereby body
size decreases with latitude.

Figure 5.

Cryptic colouration in the peppered moth, Biston betularia. In pristine habitats, like Dorset (England), the carbonaria form stands out against the lichen‐covered tree bark. However, in polluted areas, such as Birmingham (England), the typica form appears more visible and, thus, more susceptible to visually guided predators, for example, insectivorous birds. The
arrows point to the cryptic forms (from Kettlewell, ).

Figure 6.

Evidence for ecological character displacement in Darwin's finches, as plotted on histograms for beak depths for two species
of finches (Geospiza fuliginosa, light bars, and Geospiza fortis, dark bars) on three different islands. Beak depths on Santa Cruz Island, where both finches co‐occur, appear markedly different.
However, beak depths appear less extreme and have overlapping distributions where the two species occur alone, that is, on
different islands.

References

Ashton KG,
Tracy MC and
de Queiroz A
(2000)
Is Bergmann's rule valid for mammals?
American Naturalist
156:
390–415.